The present invention relates to a sputtering target which is used to form Co-alloy magnetic layers for a magnetic recording medium by means of sputtering, and also relates to a method of producing the same.
A magnetic disk unit has at least one magnetic head, which faces a magnetic recording medium with a close gap, to perform recording and reproducing by conducting magnetic recording on the magnetic recording medium and reading magnetic information recorded on the medium. The magnetic recording medium has a magnetic alloy film, made of Co-Ni-Cr, Co-Cr-Ta, Co-Cr-Pt, or the like, formed on a substrate of the medium directly or via a primary-coated film. Recently, various ideas such as increasing the coercive force of the magnetic film or reducing a noise have been proposed for the magnetic recording medium to cope with high density recording.
In a publication of JP-A-1-25617, for example, a magnetic recording medium having a magnetic alloy film formed by an alloy of four elements such as Co-Cr-Ta-Pt, a composition of the magnetic alloy film for the above-described high density recording, was revealed. When the amount of platinum added is within a range of 1 to 15 atomic %, as revealed in the above publication, the coercive force of the magnetic film becomes greater than approximately 1200 Oe, and a squareness ratio of the same became 0.8 or greater.
On page 23 of the 12th Academic Lecture Review Book (1988) of the Institute of Applied Magnetism of Japan (transliteration), it is stated that a saturation magnetization of a magnetic alloy film has not been reduced much, but coercive force of the same has been improved by further addition of vanadium to the alloy composition 20 atomic % of nickel, 10 atomic % of chromium, and balance of cobalt. It is also stated that the coercive force of the magnetic alloy film has been improved by from 1000 Oe to 1500 Oe by adding 1 atomic % of vanadium to the composition thereof.
From the foregoing, it is understood when vanadium is added to the composition of the magnetic film made of Co-Ni-Cr, the coercive force of the film is improved without reducing much saturation magnetization.
A conventional method of obtaining the magnetic alloy film is introduced in the above-mentioned Review Book of the Institute of Applied Magnetism of Japan, wherein an alloy film was formed by a magnetron-sputtering apparatus by using a so-called composite target in which pellets of nickel, chromium, and vanadium were set on a Co-Ni alloy.
However, when this type of target is used for magnetron-sputtering, a difference in a sputtering speed occurs between pellets. Therefore, there has been a problem such that geometrical irregularity or steps are generated at boundaries between pellets causing the abnormal discharge, which eventually causes a fluctuation of the film thickness and reproducing outputs. In addition, the occurrence of the abnormal discharge has an unfavorable phenomenon in that it may also cause an adhesion of foreign particles (generally called as particles) in the magnetic film to increase the number of errors occurred on a magnetic disk.
Further, when an alloy of Co-Ni or Co-Ni-Cr, which becomes a base of the above-described composite target, is produced by means of conventional processes of vacuum-melting and subsequent hot rolling, crystal grain having an average diameter of only approximately 400 .mu.m can be obtained.
The sputtering speed also depends on the crystal orientation of crystal grain of the target composition. Therefore, when the crystal-grain diameter is large, the steps are produced at the boundary area of crystal grain. These steps also cause abnormal discharge in the same manner as described before; therefore, a target having a smaller crystal-grain diameter has been desired.
In addition, Co-Ni alloy system is a magnetic substance of fcc-structure having a higher maximum magnetic permeability; therefore, it is difficult to obtain a leakage magnetic flux to a target surface, which is indispensable to magnetron-sputtering. Consequently, there is a demand to develop a target which can secure a sufficient leakage flux on the target surface.